I installed Ubuntu 12.04 on my desktop with two screens, but one of the screens has not been detected. As far as I understand the problem comes from the wrong video card driver, so I followed the instructions to install the video driver for Radeon HD2400. As a result I have the following:

In system details I see that the graphics is VESA: RV610 (which is correct), but the fglrxinfo command gives me the following:

X Error of failed request: BadRequest (invalid request code or no such operation)
Major opcode of failed request: 153 (GLX)
Minor opcode of failed request: 19 (X_GLXQueryServerString)
Serial number of failed request: 12
Current serial number in output stream: 12

And when I try to launch Catalyst control centre I get:

There was a problem initializing Catalyst Control Center Linux edition. It could be caused by the following.

There isn't any installed AMD graphics driver, or the AMD driver is not functioning properly. Please install the AMD driver appropriate for you AMD hardware, or configure using aticonfig.

In the additional drivers menu I see that "No proprietary drivers are in use on this system."